1. Field of the Invention
The present invention generally relates to a trimming circuit for adjusting a combined resistance value, in which a resistance value of a reference resistor is combined with a resistance value obtained by cutting a fuse(s) in the trimming circuit, to a target resistance value and a semiconductor device using the trimming circuit.
2. Description of the Related Art
Conventionally, as a resistance value adjusting circuit which adjusts a resistance value formed in a semiconductor device to a target resistance value, trimming circuits shown in FIGS. 1 and 2 have been frequently used.
FIG. 1 is a conventional trimming circuit diagram using fuses. As shown in FIG. 1, a trimming circuit D1 is disposed between a resistor R0 connected to a terminal P1 and a resistor R3 connected to a terminal P2. The trimming circuit D1 provides resistors R1 and R2, and fuses F1 and F2. Each of the fuses F1 and F2 is cut by a high energy beam such as a laser beam, or the fuse F1 is cut by a large current supplied between terminals T1 and T2, or the fuse F2 is cut by a large current supplied between terminals T2 and T3.
Between the terminals P1 and P2, a combined resistance value before cutting the fuses F1 and F2 is “R0+R3”. When the fuse F1 is cut, the combined resistance value between the terminals P1 and P2 is “R0+R1+R3”. When the fuses F1 and F2 are cut, the combined resistance value between the terminals P1 and P2 is “R0+R1+R2+R3”. Therefore, in the trimming circuit D1 shown in FIG. 1, since the combined resistance value between the terminals P1 and P2 can be increased by the trimming, when the combined resistance value is adjusted to be increased, the trimming circuit D1 shown in FIG. 1 is used. Therefore, the combined resistance value between the terminals P1 and P2 before adjustment by the trimming must be less than a target resistance value to be adjusted.
FIG. 2 is a conventional trimming circuit diagram using Zener diodes. As shown in FIG. 2, a trimming circuit D2 uses Zener diodes Z1 and Z2 instead of the fuses F1 and F2 shown in FIG. 1.
In FIG. 2, resistors R1 and R2 are connected in series, and a Zener diode Z1 is connected with the resistor R1 in parallel and a Zener diode Z2 is connected with the resistor R2 in parallel. The voltage drop in the resistor R1 does not exceed the breakdown voltage of the Zener diode Z1, and the voltage drop in the resistor R2 does not exceed the breakdown voltage of the Zener diode Z2.
The Zener diode Z1 is broken by having applied a high reverse bias voltage between terminals T1 and T2 so that the circuit between the terminals T1 and T2 becomes a short circuit. The Zener diode Z2 is broken by having applied a high reverse bias voltage between terminals T2 and T3 so that the circuit between the terminals T2 and T3 becomes a short circuit. In addition, each of the Zener diodes D1 and D2 can be broken by zapping.
The combined resistance value between the terminal P1 and P2 before breaking the Zener diodes Z1 and Z2 is “R0+R1+R2+R3”. When the Zener diode Z1 is broken, the combined resistance value between the terminal P1 and P2 is “R0+R2+R3”. When the Zener diode Z2 is broken, the combined resistance value between the terminal P1 and P2 is “R0+R1+R3”. When the Zener diodes Z1 and Z2 are broken and become short circuits between the terminals T1 and T3, the combined resistance value between the terminal P1 and P2 is “R0+R3”. Therefore, in the trimming circuit D2 shown in FIG. 2, since the combined resistance value between the terminals P1 and P2 can be decreased by the trimming, when the combined resistance value is adjusted to be decreased, the trimming circuit D2 shown in FIG. 2 is used. Therefore, the combined resistance value between the terminals P1 and P2 before adjustment by the trimming must be greater than a target resistance value to be adjusted.
In the trimming circuits shown in FIGS. 1 and 2, the difference between the combined resistance values before and after the trimming is large.
In addition, as shown in FIG. 2, when the combined resistance value before adjustment is greater than that after the adjustment, since a large current flows into the circuit before the trimming, current capacity of a semiconductor device such as a transistor which is connected to the trimming circuit must be large. Consequently, the size of an IC chip including the semiconductor device must be large. In order to solve the above problem, a trimming circuit shown In FIG. 3 is disclosed.
FIG. 3 is a conventional trimming circuit diagram which is disclosed in Patent Document 1. As shown in FIG. 3, a trimming circuit D3 uses resistors R1 and R2, a Zener diode Z1, and a fuse F1. In the trimming circuit D3, the resistors R1 and R2 are connected in series, the Zener diode Z1 is connected with the resistor R1 in parallel, and the fuse F1 is connected with the resistor R2 in parallel. The voltage drop in the resistor R1 does not exceed the breakdown voltage of the Zener diode Z1.
When a high reverse bias voltage is applied between terminals T1 and T2, the Zener diode Z1 can be a short circuit. When a large current is supplied between terminals T2 and T3, the fuse F1 can be cut.
The combined resistance value between the terminal P1 and P2 before adjustment by the trimming is “R0+R1+R3”. When the Zener diode Z1 is broken, the combined resistance value between the terminal P1 and P2 is “R0+R3”. When the fuse F1 is cut, the combined resistance value between the terminal P1 and P2 is “R0+R1+R2+R3”. When the Zener diode Z1 is broken and the fuse F1 is cut, the combined resistance value between the terminal P1 and P2 is “R0+R2+R3”.
Therefore, in the trimming circuit D3 shown in FIG. 3, when a combined resistance value before adjustment is greater than a target resistance value to be adjusted, the combined resistance value can be decreased by breaking the Zener diode Z1. When the combined resistance value before adjustment is less than the target resistance value to be adjusted, the combined resistance value can be increased by cutting the fuse F1. Therefore, the combined resistance value between the terminals P1 and P2 can be close to the target resistance value.
[Patent Document 1] Japanese Laid-Open Patent Application No. 2002-26258
However, in the trimming circuit using the Zener diode shown in FIG. 3, a terminal for applying the high reverse bias voltage to the Zener diode Z1 is required. When the number of the Zener diodes is increased in an IC chip, the number of the terminals is increased. Therefore, when high accuracy in the trimming is required, the area for the terminals is increased and the cost of the IC chip is increased.
In addition, since the high reverse bias voltage is applied to the Zener diode to break the Zener diode, a circuit for preventing influence of the high reverse bias voltage on an internal circuit connecting to the trimming circuit must be provided. When the size of the Zener diode is small, the high reverse bias voltage can be lowered, the influence on the internal circuit can be prevented, and the number of the Zener diodes per area can be increased.
However, even in the above case, the Zener diode may be broken by a surge voltage caused by static electricity via the terminal for applying the high reverse bias voltage and the adjustment by the trimming may be changed. In addition, since dispersion of residual resistance values of the broken Zener diodes has several 10 KΩ, it is difficult to obtain high accuracy in the adjustment by the trimming.